A relative clause is an example of a subordinate clause: "My book, which is lost, contains my homework".
Subordinate clauses provide extra information and contain a subject and a verb. The relative clause will begin with a relative pronoun such as "which", "that" or "who". For example, in the sentence above, the main clause is "My book contains my homework". This can also be thought of as the basic sentence. The relative clause in this example is "which is lost", a clause referring back to the book. By adding the relative clause we now know that the book containing the homework is actually lost.The order of clauses depends on the emphasis of the sentence. It would be equally possible to write the sentence in this manner: "My book, which contains my homework, is lost". In this case the main clause is "My book is lost", with the relative clause being "which contains my homework". Relative clauses should be marked off with commas.
Phrases, unlike clauses, do not contain a subject and verb, but instead add a cluster of information. In the previous sentence "a cluster of information" is a phrase, rather than a clause.
Test your knowledge of relative clauses with this quiz on the subject.